The proton or hydronium ion conducting membranes that are currently used in polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are typically linear or comb polymers that have sulfonic, carboxylic, or phosphonic acid groups located at the end of short branches extending from a fluorocarbon or hydrocarbon polymer backbone. In a fuel cell, these membranes are coated with an anode and a cathode on opposite sides, and then stacked between bipolar plates. In the polymer electrolyte membrane fuel cell, hydrogen gas and air are passed over the anode and cathode respectively to generate electricity. In the direct methanol fuel cell, a dilute solution of methanol is used as fuel.
In conventional membranes the terminal acid groups self-organize into hydrophilic domains that are delineated by the hydrophobic fluorocarbon or hydrocarbon backbone of the polymer and form channels that contain water. The ionic conductivity of these membranes depends on the number of acid sites per unit weight of the polymer, and on the ability to maintain the proper water content within the polymer membrane. More acid sites provide more protons or hydronium ions to enhance the ionic conductivity. Because proton transport is facilitated by water, higher water content within the membrane also translates into higher conductivity. However, if the water content of the membrane becomes too high, it will lead to swelling of the membrane and a subsequent loss of mechanical strength. Conversely, if the water content of the membrane is too low, then this will lead to a loss in ionic conductivity. This creates a problem when trying to operate these membranes at temperatures above 100° C., which is where one would like to operate a fuel cell to limit the negative effects of carbon monoxide on the fuel cell performance. As the temperature is increased above 100° C., the water content in the polymer decreases and the conductivity drops off drastically. In addition, these membranes suffer from methanol crossover, the diffusion of methanol through the membrane with the water present in the membrane, when used with DMFCs. The performance of DMFCs is severely limited by methanol crossover through the polymer electrolyte membrane, where as much as 40% of methanol can be lost as it diffuses from the anode to the cathode compartment of the fuel cell.
Methanol crossover arises because methanol is readily transported from the anode to the cathode through the hydrophilic channels within the proton-conducting membrane by bulk diffusion. Methanol is also transported as part of the solvation shell around the proton (electroosmotic drag). The ratio of methanol molecules to water molecules solvating the proton is identical to their concentration ratio in solution. Thus, electroosmotic drag increases as the methanol concentration in the fuel increases. Bulk diffusion and electroosmotic drag of methanol are responsible for the sharp decline in DMFC performance at elevated methanol concentrations. Reduction of electroosmotic drag and bulk diffusion would allow the use of higher concentrations of methanol, which would increase the fuel efficiency of the DMFC power source.
The problem of methanol crossover is further complicated because the current efforts in membrane development remain focused on increasing power densities and mechanical durability while decreasing the acid-equivalent weight (EW) and membrane thickness. The EW number is a good measure for the ionic conductivity of the polymer. It is defined as the molecular weight of polymer per acid group. The lower the EW number, the higher the acid density on the polymer and the higher the proton conductivity. For example, an EW of 1100 means that for every mole of sulfonic acid there are 1100 grams of fluorocarbon polymer backbone. Commercially available NAFION (perflurosulfonic acid polymer) is typical of the perfluorinated ionomer membranes used in practical fuel cells. The membrane consists of a fluorinated polymer backbone with strongly acidic functional groups attached to the polymer chain. NAFION membranes have relatively high EWs and low specific conductivities (1100 and 0.081 Ω−1cm−1, respectively, for NAFION 117). For comparison, the similar Dow® membranes are somewhat better, with EWs of 800 and 850 for specific conductivities of 0.20 and 0.12 Ω−1cm−1, respectively. Therefore, decreasing the EW results in markedly higher proton conductivities while a thinner membrane reduces ionic resistance. These factors together yield an increase in the DMFC power density and overall membrane performance. However, reductions in the EW have been accompanied by an increase in methanol crossover, and thinner membranes tend to exhibit reduced durability with an increased risk of methanol crossover.
The elimination or reduction of methanol crossover in a fuel cell would decrease the size of a fuel cell stack needed for a mobile power source, decrease the loss of fuel, and allow the use of higher concentrations of methanol in the fuel cell. With presently used proton conducting membranes, any decrease in methanol permeability also correlates with decreased proton conductivity.
Thus, a need exists for a durable proton conductive membrane having a low EW and high resistance to methanol crossover.